1. Technical Field of the Invention
The present invention relates generally to the radio transmitter field and, in particular, to a method and system for upconverting Direct Digital Synthesis (DDS) signals to a radio frequency (RF) band.
2. Description of Related Art
One aim in developing new radio transmitters is to make them smaller and cheaper to produce, while providing at least the same or better performance than prior transmitters. One way to achieve this goal is to move the digital domain closer to the transmitter's output and thereby eliminate a number of analog components. Transmitters that employ DDS to generate signals have been developed to achieve this goal.
Today's DDS technology has begun to mature and is now commonly used for generating communications signals. For example, DDS-driven Phase Locked Loops (PLLs) are being used in the process of upconverting Intermediate Frequency (IF) signals to the RF transmit band. Also, DDS baseband signals are being directly converted to signals in the RF transmit band. The primary advantage of using DDS-driven upconversion processes is that frequency adjustments can be made instantaneously, which is especially advantageous for multi-carrier transmitter applications. In DDS-driven PLL applications, the frequency adjustment rates are limited by the PLLs' response times. Consequently, frequency adjustments for the DDS-driven PLL upconversion approaches being used are not as instantaneous as the DDS direct upconversion approaches being used.
DDS technology can be used for just about any type of conventional modulation or access technique, such as, for example, amplitude modulation (AM), phase angle modulation (PM), Code Division Multiple Access (CDMA), etc. However, a drawback of the existing DDS technology is that the currently available digital-to-analog converter (DAC) clock frequencies limit DDS applications to the utilization of very small parts of the RF bands. In order to realize fairly good image filtering, the ratio of the IF band, df.sub.IF, to the mean IF, f.sub.IF, selected should be very low. Consequently, the DDS frequency generator used should have a very high clock frequency in order to be able to utilize it for the higher frequencies. As such, if a conventional DDS frequency generator is used to generate frequencies which are higher than up to 33% of the DDS clock frequency, signal generation problems are encountered due to the aliasing bands that are folded in from multiples of the DDS clock frequency, f.sub.clock. Furthermore, the use of conventional DACs for converting DDS signals produces relatively low signal-to-noise ratios (SNRs), Spurious Free Dynamic Ranges (SFDRs), and Effective Number of Bits (ENOBS) at the higher sampling rates, because the conventional DACs lose sufficient linearity at high sampling rates. However, as described below, the present invention successfully solves these problems.